Automatic control system for dirigible craft



April 27, 1943- H. L. HULL 2,317,383l

AUTOMATIC CONTROL SYSTEM FOR `DIRIIIGIBLE CRAFT 45 #5f INVENTOR 42 4/ HARM/:Ra L Hau. 37 SW1P Y April 27, 1943.

AUTOMATIC CON TROL SYSTEM FOR DIRIG'IBLE CRAFT Filed Jan. 27,' 1940 2 Sheets-Sheet 2 empa'.

Samp

L INVENTOR imm/AR@ L.. flu/ 1 Patented Apr. 27, 1943 AUTOMATIC CCNTROL SYSTEM FOR DIRIGIBDE CRAFT Harvard L. Hull, Leonia, N. J., assigner to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Application January 27, 1940, Serial No. 315,913

26 Claims.

This invention relates to automatic steering and control systems for airplanes or other dirigible craft, in which a direction maintaining instrument which may be a magnetic compass or a gyroscopically stabilized device is employed as a controlling member.'

The course or attitude of such craft is commonly controlled by positioning a rudder or other movable surface, which, when angularly displaced from a central position, is acted upon by an unbalanced fluid pressure and causes a turning moment to be exerted upon the craft. For automatic control .a moment in the proper direction to return the craft to course isapplied when comparison with the directional instrument indicates deviation from course. According to the present invention this is effected by displacing a control surface from its central position by an amount which causes the pressure exerted upon the surface to be primarily proportional .to the angle of deviation or departure of the craft.

Since pressure on a surface displaced with respect to the longitudinal axis of a craft is dependent f not only upon ,-the magnitude of the displacement but also upon the speed of the craft, a control system of this type has the desirable property of applying more 'correction at low speeds and less correction at high speeds than a system in which displacement of the control surface alone and not the pressure or force on the surface is made 4proportional to departure from course.

'An object whose angular position is controlled by a follow-upvsystem which applies a turning moment proportional only to departure from a reference'position will exhibit a tendency to overvshoot its proper position of rest and tov hunt about that position. To prevent this, the present invention provides means for modifying the applied torque .by adding to the primary control term, which is proportional to departure, a secondary term proportional to the rate of change any tendency of the craft to hunt by providing a damping force. v

Since a compass or other sensitive controlling element cannot directly apply suicient torque lustration in certain parts of this specification` to operate a rudder or similar control surface, where automatic control isirequired a servo sys- `tem is of necessityemployed to increase the available torque and this torque amplifying servo system may itself exhibit a tendency to hunt and oscillate the rudder which-is distinct from the tendency of the craft to hunt about the course.I

the rate at whichthe primary signal varies. By

this type of control, hunting of the servo system andmscillation of the connected rudder is prevented in a manner similar to that in which hunting of the craft is prevented.

The means lfor generating a rate signal and combining it with a displacement signal and .the means for generating a repeat-back signal may take a variety of forms within the scope of the invention. A preferred form is illustrated in which all such means are electrical, resulting in compactness and the elimination of mechanical connections between the transmitting and receiving stations of the control system.

It is to be noted that while for purposes of i1- and in the claims the use of the improved system of the invention for azimuth controlvis referred to, it will readily be vunderstood that in the case of aircraft the system is ,equally applicableV (and such use is contemplated) to position an'elevator or aileron surfaces to control angular motion about the lateral and/or longitudinal axes of the craft, the ,equivalent of the azimuth deviation signal being generated by angular displacement from reference positions relative to these axes.

One 'object of thefinvention is to provide an anti-hunt steering or control system for a dirigible craft.

Another object is to provide means for applying a corrective turning moment to a yawing craft which is proportional to deviation from course and to a time derivative or derivatives of such deviation. l

A further object is ,to provide a control system in which a sensitive directional element controls the course or attitude of a craft without hunting by means of a servo system which itself operates in a dead beat manner,`

Still another object is to provide an electrically the motor input circuit.

2 controlled hydraulic servo system with electrical repeat-back responsive to hydraulic pressure.

In the drawings,

Fig. 1 is a schematic diagram of one form. of

control system according to the invention in which rate generating circuits are associated with a signal generator at the compass and with tion as a gyroscopic compass or directional gyro-l scope having arotor 2 spinning about a horizontal axis and a follow-up member or phantom" I the vertical plane of which is maintained in fixed azimuth relationship to a vertical plane normal to the spin axis of the rotor by driving motor l ycontrolled by follow-up amplifier l. A suitable `type of follow-up amplifier and associated apparatus for causing motor 4 to maintain phantom" .3 in alignment with rotor 2 is shown in U. S. Patent 2,139,558, issued December 6, 1938, to F. L. Moseley. W. T. Cooke and C..A. Frische.

When a deviation of the fore and aft axis oi' the craft from the set course occurs. rotor 2 is aamsss 17, 1937, now U. S. Patent No. 2,233,415, in which the use of a parallel resistance-capacity circuit in sexies with a resistance as'a differentiating and combining device forpreventing hunting of a follow-up system is described and the relation of the circuit constants discussed.

Added to the potential across resistor Il'is the potential derived from repeat-back potentiometer I8 which has a `movable contact I0 actuated by differential pressure relay A2II through rack and pinion 2|. Battery V22 is connectedacross the `resistance winding ofv potentiometer I8 and a mid tap of this winding is connected to one terminal of resistance IJ. A -voltage proportional to the diterence in between. the pressures .acting on the two sides of the resilient -diaphragm 20' of relay 2l is taken from potentiometer Il as contact Il is displaced in response to exure of the diaphragm and this voltage is applied in series with the potential across resistance I1 in a sense to oppose the potential difference thereacross.

The pressures acting on'the opposite faces of diaphragm 2II' are those existing on the opposite ends of piston 2l o f hydraulic motor 2l which are transmitted through connecting lines Il, 2l

- andu, 2l' respectively. Piston 2l is operatively displaced relative to the housing of the compass and motor l, actuated by follow-up amplifier B, Vrotates phantom I by means of worm drive 8 to maintain the fixed relation of this member to rotor 2. The rotation of-phantomV is transmitted by gear B on the "phantom" shaft to ring gear 1 constituting one arm of a mechanical differential B. a second arm oi which is movable by means of course setting knob l through wormand wheel drive III and the third arm of which rotates movable contact II of compass potentiometer I2. The resistance winding of potentiometer I2 is connected across battery I3 and a mid tap of said winding is connected ,to parallel circuit I4 consisting of condenser Il and resistance I8. When contact II is in its central position, no voltage is -applied to circuit I4 but as this contact is displaced to one side or the other of its central position a voltage is applied to circuit I4 proportional'to the angle through which contact II is rotated and whosesense or polarity is determined by the direction in which the contact moves from its central position.

'I'he current through circuit I 4 divides into two parts corresponding to the respectivepaths through condenser Il and resistance Il, and a voltage drop across resistor I 'Il is produced by this current proportional to its two components.

'I'he series circuit comprising condenser Il and resistance II acts as a differentiating device and the component of the potential across resistor connected to rotate rudder surface 28, and thev net force resulting from a difference of pressure on'the two ends of piston 2l is balanced by the net force exerted upon rudder 20. The differential pressure on the piston is therefore proportional to the resultant fluid pressure on the rudder. It will be apparent'then that the reversible potential derived from potentiometer I 8 by the displacement of contact I9 lis a pressure repeatback signal since itis proportional to the sure on the displaced rudder surface.

The algebraic sum of the deviation. rate oi deviation and pressure repeat-back signals is ap plied as a potential to circuit 2'I consisting of condenser 2l and resistance 2l connected in parallel. Resistance 29 passes a current 'proportional to the applied potential, while condenser 28 in cooperation with resistance 3| dinerentiates this potential, the sum of the'cur'rents in the two branches causing a difference of potential across center-tapped resistance II' which is applied to the grids of electron tubes 20 and l0', connected in a balanced circuit and negatively biased to actas rectiilers by grid battery il.

` The plates of tubes and Ill'are connected to o oppositely poled field windings (not shown) of I1 due to the current through the condenser isv proportional to the rate of change of potentialy applied to circuit I4. `The current through rei sistance Il produces a'potential component across I1 which is in phase with the -input potential. In this manner a difference of potential exists across resistor i1 proportional to-departure from course or yaw and to the rate f change of departure or yaw.

reversible motor t2, which may be of any suitable type. by leads I2'. The common connection or Junction'of the fields of this motor is connected to one end of the secondary winding vof supply transformer 38. the other end of which is connected to the cathodes of the tubes. Iransformer Il is fed from a suitable A. C. supply. By this arrangement, a potentialdinerence oi` one polarity acrossthe grids of tubes l0 and III' causes rectified current to flow throuh one of the iieldv windings and rotate motor I2 in one`- direction while a signal of the opposite polarity causes current to now in the other winding and rotate the motor in the opposite direction. Motor I2 may."

\ for example, be of a series type.

'Iheprinciple of the anti-hunt feature of the V Y servo motor control circuit as represented by circuits Il and 2l has been disclosed in my c opending U. S. application, Serial No. 148,653. illed June l to valve pistons I3 and Il' Motor l2 is connected by rack and pinion Il' oi' valve III arranged to differentially control the openings of ports in pressure lines ll and-I4'. respectively, leading fromA opposite ends of *the cylinder of hydraulic motor 24. A two unit pump 3l. driven by motor Il, maintains a continuous flow of duid from sump 31 through two parallel paths. one

pres- "the craft is displaced with ment causesV a displacement I potential is applied to circuit 34 and 34' respectively, resulting in equal'pressures on the -two ends of piston 2l, under which condition no torque is exerted on rudder 23 and this surface is centralized. When pistonsy 33 and 33f are*l displaced by the rotation of motor 32, the port openings are changed differentially, one port being further uncovered and the other restricted causing differential changes of pressure on the two sidesof piston 23 and thereby causing this piston to dispiace rudder 26 until the pressure `exerted on this control surface is proportional to and balances the net pressure on piston 23.

Throttling valve 4I in the common return line is adapted to control the stand-by pressure of the system. It is desirable to' provide means for hydrostatically balancing the pistons which may be done by by-passes 43 and 43'.

The operation of the system shown in Fig. 1 is as follows: While the ship is proceeding along the set course, the vertical plane of phantom element 3 of lcompass I is stationary and, for example, aligned with a vertical plane normal to the spin axis of rotor 2 and no unbalanced fluid pressure is exerted upon rudder 26. No electrical signal is then derived from potentiometers I2 and I8 and none is applied to the grids of the tubes 30, 30'. When departure from course ocfcurs, the housing of compass I which moves with respect to rotor 2. Phantom'e1ement 3 is then moved by `motor' 4 into alignment with the rotorand as' this moveof contact arm II central position, a I4 and a potential appears across resistor I1 proportional both to the angle of departure and to the rate of change of that angle. This signal potential is applied to the grids of tubes 30,30' andthe resulting differential change of plate currents of these tubes causes motor 32 to be operated in one direction or theother and thereby displace the pistons of dual valve 43 and differentially change the ports openings in this valve. The differential change of port opening causes unequal pressures to be exerted upon' the two ends of piston 23 which is thereby caused to move rudder surface 26`in a direction to return the craft to its course. The unequal pressures on piston 23 are also applied to diaphragm 23' of pressure I connecting pressures lines and the resulting displacement of this diaphragm causesmovable contact arm I9 of potentiometerfl to be displaced of potentiometer I2 from its from its central `position and. tap off a repeatv back potential from potentiometer I8 which opposes the primary signal across resistor I1. This repeat-back signal therefore decreases the net Vsignal applied to the grids of tubes 3Q, 30', andl eventually reduces Vit to zero. Thev 4rudder as it moves from its central position experiences an increasing unbalanced fluid pressure and the relation between rudder movement and repeatback signal may be adjusted; so that a deviation of the craft from course will cause any desired pressure to be exerted on the rudder at equilibreiay by way ofv erates a. primary signal which will rium and hence any desired turning moment to be exerted on the craft. l

In the above discussion the-effect the ship yaws from the course and. causes a `signal to be generated by potentiometer I2, the rate of change of this signal is generated by the' cooperation of condenser Il in circuit I4 and resistance l1 and added tov the primary signal thereby speeding up the turning of rudder 2l. As the ship returns to its course and the sign of the rate signal is reversed, the rudder is centralized sooner than would be the case if the primary signal were acting alone. If there is any tendency to overshoot the course, the primary and rate signals, both acting in the same direction after the course line is passed to apply reverse rudder.v

oppose'such tendency. i

While the differentiating and combining means including circuit I4 acts to prevent hunting of the craft about the course, the means of which circuit 2l' is a part is designed to prevent hunting of the servo system and consequent oscillation of the rudder which may occur at a relatively higher frequency. In circuit 21, the charging current of condenser of change of the signal applied to the condenser as a voltage drop across resistance 3|' and the combination of said signal and its rate applied to tubes 30, 30' prevents servo system or internal.

hunting by modifying the operation of motor 32 to overcome the effect of the inertia of the connected moving members.

For setting a course,knob 9 is turned until the desired course is read on dial 41 by meansof index 48. Rotation of this knob moves contact II of potentiometer I2 through differentialf l without disturbing the position of "phantom 3 owing to non-reversible worm drive 3. .This genonly be balanced when the craft has turned through an angle which causes motor 4 to realign phantom 3 and so return contact II to its central position.

The system of Fig. 2.which is a modification in several respects of that shown in Fig. -l employs a rate generating circuit for obtaining the rate ofgchange of the-repeat-back signal and the combination of repeat-back signal and itsv rote offs:-

change opposes the combination of primary signal and its rate. The relation of this system to the one shown in Fig. 1 is best seen by considering the mathematical relationship of the several control terms. If the primary signal, proportional to angular departure from course, is represented by the symbol a and the repeat-backand the rate .of change of the repeat-back' signal by ySecond order rates or derivatives will be represented by (P0 (Pd: l j

it; and -d-t; i A

In the system according to Fig. 1 the potential ofthe devia-- tion rate signal has sovfar been' neglected. As

28 generates the rate l as the rate of did'erence 4- g applied to the mds or cubes n, se' has co ponents proportional to 0, o;

i! Q d: di

d20 Mld d t, Resistors il/and 2l pass the 0 and components unchanged. Condenser Il k,to which a potential proportional to o is applied: generates a the condensers of circuits Il and B2. The above terms may be grouped into two vdifference terms as was done in case of thecircuit of Fig. 1:

showing that the two circuits are equivalent when the yaw second derivative term is negligible.

Referring now in` d etail to the arrangements of 1'0 Fig. 2. there is shown a potentiometer 53 having term which is passed unchanged by resistance Il -while condenser 21 differentiates the 0, o `an g! t terms resulting in components in do d d'0 3T' and W The relative magnitude of th`e several components depends upon the constants of 'the circuits. For example, the rate at which a ship or aircraft yaws will ordinarily be considerably slower than the -rate at which the servo system hunts and in consequence, the capacity of condenser Il in relation g da potential generated by said condenser appears across resistor 3| As yaw and hunt frequencies i5 corresponding .elements of Fig. 1.

represent gyro horizons in which case the alignment of the follow-up element or .phantom willv a movable contact 54 arranged to be moved in correspondence with a follow-up element vof a directional instrument through .differential gearing in a manner similar to the actuation of the Directional instrument 43, if' considered to be shown in elevation, may be a compass or directional gyroscope Vas in Fig. l or ,if considered to be shown in a plan view, either or both instruments may cause a signal proportional to rotation of the craft about ahorizontal axes tobe generated.

Either the directional gyro or gyro horizon or.

both may be a part of the conventional control Vunit of an automatic pilot. Potentiometer -53 has applied to its winding an A. C. potential from one of the secondary windings 55 of transformer 56 whose primary is supplied from a suitable alternating current source...V To derive the electrical center of potentiometer 53 as a reference point there is bridged across the terminals thereof a centertapped resistor 51 so that when movable contact 54 is in its central position, no A. C. potential difference exists between this .contact and the mid point of resistor l1. When contact 54 is displaced from its central position, due to a departure from course of the craft, the A. C. potential between contact 54 and the center tap of o resistor B1, whose phase depends upon which side draw closer together the circuit can be designed to increase the usable portion of second derivative voltage which as is well'known may be employed to further decrease the tendency to hunt.

fIf the second derivative `of the yaw signal is neglected the terms representing the component potentials applied to tubes ll and 3|' may be grouped as follows: I

d, Y mi siii-'0; l, i the negative signs indicating that thepotentials are combined in' opposition. The nrst term Vreprecents the lag of the rudder`in taking a position where the fluid pressure on it is proportional tQ 5.-, transmitters are shown deviation from course and the second term rel) resents the rate at which this lag changes. .The groupingof the differenceof the two rate terms do -l and a-t d-fw-o is Justiable mathematically since both 0 and o may be expressed as functions of a single variable. t, that is, of time.

In the'A circuit of Fig. 2 the potential applied to the grids of tubes Il and Il is proportional to d0 dd a-talld an the two rates being generated independently by of its central position contact 54 rests,`is applied to demodulator or rectifier I8 of any suitable type thereby resulting in a direct current, signal of reversible polarity as a component of the output 4,-; of said demodulator. vBalanced demodulating or rectifying means for'changing reversible phase A. C. to reversible polarity DQC. are well known in the art and are shown, forexample, in U. S.Patent #2,088,654, issued August3, 1937 to the present` .5d applicant. In place of center-tapped potentiometers, other means for obtaining reversible phase A. C. signals proportional to displacement may be employed as, for example, a "Selsyn transmitter used as a signal generator. Such A. C. inductive Y in U. S. Patent #2,054,945,

issued September 22, 1936 toR. H. Nisbet.

In a similar manner, a reversible 'phase A. C.

signal proportional to the pressure on rudder surface I9 is generated by potentiometer 8l which has a movable contact 0i actuated by differential pressure relay 82 and a reversible potential D. C. signal is derived therefrom by means of a demodulator 5l. Potentiometer 80 is shown as effectively center-tapped by being ener- 05 gized from center-tapped secondary winding ll' 75 Fig. 1.

of transformer ll.- The D. C. signals fromfdemodulators I8 and` combined signal which contains components pro- `I0 portlonal to departure, rateoi' change of departure, repeat-back pressure, and rate of change of repeat-back pressure, is applied to the grids of tubes and NI' ,for controlling motor il which corresponds in 0l are separately applied to rate circuits 5I and 52, respectively, and the` function to motor I2 oi"` generator of this signal.

A variation of the hydraulic motor and control valve of Fig. 1 is shown in which motor I4 through gearing Il moves the .double piston it.

' stand-by pressures in motor 1liy may be regulated.

v The details of Fig. 2 and the features of operation not described will be apparent from the description of the system of Fig. 1. tinctive feature of the system of Fig. 2 is the position of the circuit for obtaining the rate of change oi' the rudder pressure repeat-back signal which in this case is directly associated with the The equivalence of the'arrangements of Fig. 2 and of Fig. 1 when second order derivatives are neglected, has been pointed out.

' As'manychanges could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or'shown in the accompanying drawings shallbe interpreted as illustrative and notV tion maintained by said instrument, a rudder,

means for returning the craft to, the set course responsive to departure therefrom including a servo system operatively connected-to position said rudder, and a controller for said servo sys-x tem continuously actuated in accordance with a combination of the force due to fluid pressure on said rudder and the rate of change of said force. A Y

2. In automatic steering apparatus for dirigible craft, a direction maintaining instrument, a rudder, and anti-hunting motive means for positioning said rudder to steer a course substantially in Vfixed relationship to the direction maintained by said instrument, including, a servo system for actuating said rudder, means for controlling said servo system in accordance with departurefrom course and in accordance with a time derivative thereof, and further means for continuously applying a controlling input to said servo system in accordance with the force due to uid pressure upon said rudder and a time derivative of change of said force.

,3. In automatic steering apparatus for dirigible craft, adirection maintaining instrument, means for steering a yawing craft along 'a mean course settable with respect to the direction 'maintained by said instrument, including course setting means, a rudder, means responsive to yaw for positioning said rudder to exertl a correctiveturning moment on the craft substantially proportional to yaw and to rate of change of yaw, said last means including' a rudderactuating servo system and repeat-back means suitable for continuously controlling said servo system in opposition to said yaw responsive control in accordance with a combination of the The dis-Y cordance with said combined signal and the rate force due to fluid pressure exerted uponl said rudder and the rate of change of said force.

l4. In automatic steering apparatusfor dirigible craft, a direction maintaining instrument, means for setting'a course relative to the direction maintained by said instrument, a rudder, a servo system for positioning said rudder, means for generating an electric signal corresponding to 'a control term having com ponentsV respectively continuously proportional to ldeparture from course, rate of change of departure, rudder pressure and rate of change of rudder pressure, and electrical means for controlling said servo system in accordance with said signal to malntain the craft on course without hunting. t

5. In automatic steering apparatus for dirigible craft, a direction maintaining instrument, means for setting a course relative to the 'direction maintained by said instrument, a rudder, a

hydraulic servo system for positioning said rudder, means for generating an electric signal corresponding to a control term having components respectively and continuously proportional to departure from course, a first order time derivative of change of departure, a higher order time derivative of changeof departure, rudder pressure and rate of change of rudder pressure, and electrical means for controlling said servo system in accordance with said' signal to maintain the craft on course without hunting.

6. In automatic steering apparatus for dirigible craft, meansfor setting a course, a rudder, anti-hunt motive means therefor, and control apparatus for said motive means operative for maintaining the craft on a set course includingY a servo system for positioning said rudder, means for obtaining a control\ signal proportional to departure from course, means f or obtaining a control signal proportional to rate of change of departure, means for obtaining a control signal continuously proportional torudder pressure, means for combining said several signals, and means for controlling said servo system in acof changeothereof.

'1. Automatic control apparatus for dirigible craft comprising, a displaceable surface controlling rotation of the craft about one of its three derivative of said signal, means for combining said primary signal and its derivative, means for obtaining an electrical repeat-back signal proportional to fluid pressure on said surface, means for obtaining a signal proportional tothe difference of said repeat-back signal and the combination of said primary signal and its derivative, electrical means for obtaining a time derivativeof said difference signal, means for combining said difference signal and its derivative and a servo systemoperatively connected to displace said surface in accordance with said last composite signal for maintaining the craft with said second axis in the set direction without internal or external hunting.

8. Automatic control `apparatus for dxflgible` craft comprising, a displaceable surface controlling rotation of the craftv about oneof its three mutually perpendicular principal axes, means for maintaining a reference direction in space,

- change.

means for setting the mean position of a second principal axis relative to said direction, means for obtaining an electrical signal proportional to deviation of said second axis from said mean position, electrical means for obtaining`r a time derivative of said signal, means for obtaining a composite signal proportional to the algebraic sum of said primary signal and its derivative,

means for obtaining an electrical repeat-back signal proportional to fluid pressure on said sur-v face, electrical means for obtaining a time derivative of said repeat-back signal, means for obtaining 'a composite signal proportional to the Y algebraic sum of said repeat-back signal andits derivative, a servo system operatively connected to displace said surface, and means for controlling said servo system in accordance' with the v difference of said twocomposite signals to maintain the craft with said second axis in the'set direction without external or internal hunting.

9. Automatic control apparatus for dirigible craft in accordance with claim 4 in which the means for obtaining said departure signal 'and said rudder pressure signal include tapped potentiometers energized by a. D. C. supply for obtaining reversible polarity D. C. potentials proportional to the measure quantities.

10. Automatic control apparatus -for dirigible craft in accordance with claim 4 in which'thev means for obtaining said deviation and rudder pressure signals include potentiometers supplied from acenterftapped winding of a transformer,

said transformer being energized from an A. C. supply, andrectifying means connected to the output of each potentiometer for generating re- 11. Automatic control apparatus fordirigible Ycraft in accordance with claim "l in which the means for obtaining derivatives of the departure and repeat-back signals include a resistancecondenser network.

12. Control apparatus for dirigible craft comprising, a 'displaceable control surface, servomeans for displacing said surface, and means -adapted to control said servo means continuously in. accordance with the difference of two factors proportional respectively to the amount of departure from course and to counter pressure r on said surface, andfrther meansfor controls ling said servo means in accordance with the difference of two other factors proportional re-v spectivelyl to the rates `oi.' change of said first mentioned factors. f

'13. Automatic control apparatus for diri'gible craft comprising, electrical means for measuring departure from |course, electrical means for measuring rate' of change of departure, a reversible hydraulic motor having a movable piston actuated by differential pressure changes on opposite ends thereof, a rudder surfacelopera-- tively connected to said piston, electrical means versible polarity D. C. potentials proportional to the measured quantities.'

'a 15. In apparatus in accordance with claim 13. means for measuring the dinerential pressure on said piston comprising, an effectively tapped potentiometer having a movable contact and.

energized from a suitable potential supply, a hydraulic' pressure relay actuated by the dinerential pressure on said piston and operatively connected to displace the movable contact of said potentiometer in 'accordance' therewith to obtain a reversible polarity D. C.y potential `proportional to rudder pressure.

16. In automatic steering apparatus i'or dirigi ble craft, a direction maintaining instrument,

means for setting a-course relative to the direction maintained by said instrument, a rudder, a servo system for operating said rudder including a positionabie member controlling the force applied by said servo system, positioning means for said member, and means for controlling said positioning means in accordance with a quantity having components respectively and continuouslyproportional to departure of the craft from the course set by said first named means,

' 4vthe fluid pressure on said rudder, and the rate of change f said pressure.

17. In automatic steering apparatus for dirigible craft, a direction maintaining instrument, l

means for setting a course relative to the direction maintained by said` instrumenta rudder, a servo system for displaci g said rudder including a positionable member c ntrolling the force exerted by said servo system, a motor for displacing said'positionable member, and continuously operative means for causing said motor to operate at a speed proportional to departure from course. fluid pressure on said rudder and\r`ate of chang of said pressure.

18.v In automatic steering apparatus for dirigible craft, -a direction maintaining instrument,

means for setting a` course relative to the direction maintained by said instrument, a rudder, a

hydraulic servo system for positioning said rudder, means for generating an electric signal corresponding to a control term having a component .A signal proportional to the force exerted upon said rudder, a condenser, and means for applying said proportional torudder pressure including a potentiometer effectively center-tapped and energized from an A. C. supply together with a rectifier for securing a D, C. output therefrom.

19. In a rudder control system for dirigible craft, a rudder, a servo system for positioning said rudder, means for generating an electrical i signal Vthrough said condenser to 'control vsaid l .means furnishing a continuous measure of the servo system. Y f

20; In steering apparatus for a dirigible craft, means mounted lupon the craft furnishing a measure of departure of the craft from a predetermined course, a rudder for steering the craft, servo means for displacing said rudder, and

means for obtaining and applying to said servo means a controlling signal having continuously effective components respectively proportional tov departure from course and to the force and rate of change of forceexerted by said servo means upon said rudder.

21. In automatic control apparatus for dirigible craft, a rudderl controlling attitude oi' the craft about an axis thereof, servo means for exerting a displacing force upon said rudder,

force exerted by said servo'means, means for materializing a quantity having components continuously proportional to said measured force and the rate of change thereof,\and `means for s controlling said servo means in accordance with said last quantity.

22. In automatic control apparatus Afordirigible craft, a rudder displaceable to control the angular position of the craft about an axis thereof, servo means for displacing said rudder, means furnishing a measure of deviation of the craft from a reference position about said axis, means,

furnishing measures of the rate of change and a higher time derivative of said deviation, meansv jointly andl continuously controlling said servo means in accordance with said three measured quantities and repeat-back means for. further continuously controlling said servo means in' accordance with the force exerted thereby upon said rudder and the rate of change of said force.

23. In automatic control apparatus for dirigibie craft, a rudder displaceable to control the angular position of the craft relative to a reference position, servo means for displacing said rudder, means furnishing a measure of deviation of the craft from said reference position and for automatically controlling said servo means in accordance with the measured quantity, and means actuated in proportion to the force applied to said rudder for obtaining continuous measures of said force and the rate of change thereof and for further controlling said servo means in accordance with said last two measured quantities,

-24. In automatic control apparatus for dirio gible craft, a rudder controlling attitude of the craft, servo means for displacing said rudder, means furnishing a measure of deviation of the craft from a reference attitude, means actuated in proportion to the force applied to said rudder by said servo means for obtaining a. continuous measure of said force, means combining said two measured quantities and deriving from the combination thereof .a quantity having components respectively proportional to said combination and the time rate of change of said combination, and means for controlling said servo means in accordance with said last quantity. Y

25. In-a control system,` a dispiaceable object, servo means for displacing said object, circuit means for obtaining a primary electric signal for controlling said servo means, means for obtaining a repeat-back electric signal having continuously effective components respectively proportional to the force exerted on said controlled object by said servo means, circuit meansA for 'combining said two signal and for obtaining from the combination thereof an electric quantity having components respectively proportional to said combination andthe rate of change of said combination, and means for controlling said servomeans in accordance with said last named quantity.

26. In a remote control system, a controlling object and a controlled object, servo means for positioning said controlled object, means for obtaining a primary signal-having components respectively proportional to displacement and rate of change of displacement of 'said controlling object, means for obtaining a repeat-back electric signal having continuously effective components Arespectively proportional to the force exerted on said controlled object by said servo means and I the rate of change of said force, circuit means for combining said two signals and for` obtaining from the combination thereof an electric quantity having components respectively proportional to said combination, and means for controlling with said last 

